1. Field of the Invention
The present invention relates to an image input system that reads image information by means of photoelectric transformation and, in particular, to a light-supplying optical device used in an image input system.
2. Description of Related Art
FIG. 6 shows a conventional image input system, the light-supplying optical device of which is shown in FIG. 7. Such a light-supplying optical device can include an optical arrangement such as is shown in FIG. 8A and FIG. 8B, in which the optical path of the light is manipulated by two mirrors 102 and 105 (see FIG. 7) so as to be contained within a confined space.
As shown in FIG. 7, one of the two mirrors used to manipulate the optical path of the light is a toric mirror 102. The toric mirror 102 is comprised of a curved surface having curvature in both the lengthwise direction (i.e., the horizontal direction as shown in FIG. 6) and in a direction perpendicular to the lengthwise direction (i.e., the vertical direction as shown in FIG. 6). The arrangement is such that light produced by the light source 103 illuminates a width of one line on the document by means of the curved surface in the lengthwise direction. That is, the curvature R1 (see FIG. 8A) in the lengthwise direction causes the rectangle of light reflected by toric mirror 102 to have a certain length. The light image provided by light source 103 is formed into a linear image onto the surface of the original document by means of the curved surface in the direction perpendicular to the lengthwise direction. That is, the curvature R2 (see FIG. 8B) in the direction perpendicular to the lengthwise direction causes the rectangle of light reflected by toric mirror 102 to have a certain width, which usually is much less than the length.
As shown in FIG. 6, light emitted from the light source 103 is incident on the toric mirror 102 at an angle 111 relative to a line 110 normal to a central portion of the mirror 102. Accordingly, the light reflected by the toric mirror 102 is reflected at the angle 111 relative to the line 110. Such light is formed into an image on the surface of the original document 101 by means of a second mirror 105, which also is referred to as an optical path conversion mirror. By moving a carriage 107 that holds the original document 101 in a secondary scan direction indicated by the arrows in FIG. 6, a linear image sensor 109 successively reads image information from the entire surface of the original document (one line at a time in what is known as the primary scan direction).
The image information (i.e., data) of the original that has been illuminated by light rays is composed into an image on the linear image sensor 109 (e.g., a charge-coupled-device (CCD)) by a projection lens 108. By moving the carriage 107 that holds the original document in the direction indicated by the arrows in FIG. 6, the information on the entire surface of the original is sequentially read by means of the CCD 109.
The light source 103 can be constructed as shown in FIGS. 9A and 9B. A stem 202 is affixed to the top of the light source base 201, on the top of which stem multiple LED chips 210a, 210b are arranged in a line and bonded to its top. A conical reflector 202a (see FIG. 9B) is formed at the periphery of each LED chip 210a, 210b, and reflects light emitted in the horizontal direction so that the light radiates upward from each LED chip 210a, 210b.
The light source 103 comprises two lines of LED chips 210a and 210b in order to irradiate three colors of light. The blue LEDs, which emit a smaller amount of light, are positioned in a line (e.g., 6 LEDs) 210a, while the red LEDs and green LEDs are positioned in the order GRGGRG, for example, in the other line 210b. After the light from the LED chips 210a and 210b is reflected by the reflector 202a and radiates upwardly from each chip (FIG. 9B), the light is reflected by the blue reflecting membrane 205a or by the total reflecting mirror 205b so as to be radiated forward (to the right side of FIGS. 9A and 9B, and to the left side of FIG. 6). This light is then collected by the toric mirror 102 so as to form a line or area of light on the surface of the original (see FIG. 6).
The light from the blue LEDs is reflected by the blue reflecting membrane 205a, and the light from the red and green LEDs is reflected by the total reflecting mirror 205b. When viewed from the front direction (the right side of FIGS. 9A and 9B, and the left side of FIG. 6), the three colors appear to originate from the same position. Switching of the colors RGB is electrically regulated, making high-speed reading possible.
With the conventional illuminating optical device described above, light emitted from the light source 103 is incident at the angle 111 from the line 110 normal to the toric mirror (i.e., the optical axis of the toric mirror), while the toric mirror has curvatures R1 and R2 in the perpendicular directions as shown in FIGS. 8A and 8C, which are top and side views, respectively, of the light-supplying optical device in an unfolded state. Consequently, the angle of light reflection varies between the ends and the center of the toric mirror 102. As a result, the light source image 110a formed on the surface of the original document 101 is bow-shaped, as shown in FIG. 8B, thus creating the so-called line bow phenomenon. On the other hand, because the reading line 111a of the CCD 109 on the surface of the original document is a straight line, a problem arises in that lighting cannot be obtained that is uniform at both the center and at the ends of the reading line 111a, as indicated in the graph of FIG. 8B, which is a graph showing the amount of light received by the CCD 109 relative to positions along the CCD 109.